On the one hand side, web applications comprising sensitive data (e.g. e-commerce and e-banking applications) need to be based on a secure and reliable architecture. On the other hand side, hackers exploit software applications for self-esteem or malicious intention. Regardless of any motivation, software developers should implement security mechanisms which are able to limit malicious behaviors. The frequent and basic target field usually is a client-server architecture, where the client may be any type of application. A server, which is sometimes called a daemon, is activated and awaits a client's request to access one or more resources. Usually, multiple client programs share the services and/or resources of a common server or common server programs. For such applications authentication may be crucial for data transactions from the client to the server and the underlying resource and back to the client. Hence, authentication mechanisms are necessary, which prevent fraudulent attacks to such applications.